...does anybody know why I have only 1000Kh/s with 2 280x and using these command line -I 13 -g 2 -w 256 --shaders 2048?
on linux or windows? try to set higher
intensity ...I would start there at least; thus the last step is to flash mining-friendly BIOS

On windows
I use the same setting like yestarday when I had 1200-1250 kh/s
Might be a program running in the background.
For example, if I have flashplayerplugin (adobe flash player) running in the background I sometimes see drops of 50-100 kh/s even though I am not watching any youtube or whatever.
If I set my computer to only mine, I always close the flashplayerplugin task in task manager.